Major efforts have been undertaken in the world to develop, certify and deploy explosive detection systems. In recent years, at least 2000 to 3000 explosive incidents warranting investigation have occurred within the United States and a comparable number in other countries. As the spread of terrorism rose to an alarming proportion across the globe, the need for sensitive yet reliable detection of concealed explosives has also increased exponentially.
Further, the analysis of explosives is of great importance in a variety of fields. For example, in forensic science, identification of these explosives and their degradation products can be used to identify persons in recent contact with an explosive device. In environmental protection, knowledge of explosives present in undetonated landmines can be critical in protecting the surrounding soil and groundwater as explosives can leach into and persist in the environs and pose a threat to the living inhabitants. In industrial quality control, manufacturers of explosive materials must assure consumers that their products are safe, effective and free from contamination by monitoring the composition throughout the manufacturing process.
Explosive sensors have been developed based on a number of detection technologies, such as colorimetry, fluorescence, mass spectrometry, electro-chemical methods, and Surface Enhanced Raman Scattering (SERS), and so on. However, detection of trace quantities of explosives in the gas phase has remained a significant challenge to analytical chemistry. The low vapor pressure of most explosives, in the parts per billion (ppb) to parts per trillion (ppt) ranges at room temperature, pushes the limits for most methods including modern fluorescent techniques. See S. Yang, et al., Anal. Chem., 2014, 86, 7931-7938.
In earlier work, Applicant reported that xanthene dyes interact with a variety of explosives and related materials in dimethyl formamide (DMF) solution and these can be detected by changes in emission. See, C. A. Latendresse, et al., Anal. Methods, 2013, 5, 5457-5463. Xanthenes are readily available and inexpensive laser dyes with high quantum yields. While most of the analytes tested in our earlier work showed quenching of the fluorophore, a few of the molecules showed enhancement of the fluorescent signal, which was surprising. Except for trinitrotoluene (TNT) and trinitrobenzene (TNB), the fluorescent signal changes were modest, typically 10% or less. TNT and TNB showed large signal changes because they reacted with the DMF solvent to form highly colored products, which absorbed the emitted light.
Still, there remained a need for a more sensitive yet reliable sensor for detecting explosive-related analytes. Further, the challenge of detecting multiple analytes in their respective gas phase using strong fluorophore-based signals remained unmet.